Addition-crosslinking silicone compositions crosslink by reaction of aliphatically unsaturated groups with Si-bonded hydrogen (hydrosilylation) in the presence of a catalyst, typically a platinum compound. Owing to the fact that the crosslinking reaction commences when the essential constituents are simultaneously present, addition-crosslinking silicone compositions have hitherto been prepared virtually exclusively as two-component formulations, with the composition of the individual components being such that all three essential constituents are present together only after the components are mixed. Customarily, one of the components comprises the alkenyl-functional polyorganosiloxane and the platinum catalyst while the other component comprises the SiH-functional crosslinker, if desired in combination with the alkenyl-functional polyorganosiloxane. After mixing the individual components, complete curing to form the silicone elastomer can be carried out at room temperature, but it is customarily carried out at elevated temperature.
The two-component system for addition-crosslinking silicone compositions is associated with numerous disadvantages, for instance logistics, the high risk of contamination by traces of platinum and the presence of an additional mixing step. Although mixing of the components does give a ready-to-use composition, this composition has only a very limited pot life at room temperature. This not only makes immediate processing necessary but also requires frequent cleaning of the stock vessels, metering units, processing machines, etc., since any material remaining, for example as a result of backmixing or sticking to the walls, ultimately gels.
Owing to the disadvantages of such two-component compositions, there has been no lack of attempts to make addition-crosslinking silicone compositions available as one-component formulations. Since in the case of a one-component system all constituents necessary for crosslinking are present simultaneously, the problem is essentially to suppress premature commencement of the crosslinking reaction which normally proceeds even at room temperature. Possible ways of setting (increasing) the pot life of an addition-crosslinking composition in a targeted way have been known for some time, e.g. by the use of inhibitors which are capable of considerably reducing the activity of the platinum catalyst at room temperature, for example phosphorus compounds in combination with peroxides as described in U.S. Pat. No. 4,329,275 or azodicarbonyl compounds as described in EP-A-490 523. Although choice of type and amount of such inhibitors makes it possible to increase the pot life to any desired extent, an adverse effect on the crosslinking behavior is unavoidably associated with this increasing pot life. This is particularly true when the pot life is extended to a number of months by means of high inhibitor contents. In such cases, increased start temperature and undesirable crosslinking behavior ranging from a low crosslinking rate to insufficient crosslinking, are the result.
A further, fundamentally different technique is encapsulation of the platinum catalyst in a finely divided material which releases the platinum only at elevated temperature. This can be achieved, for example, by microencapsulation of the platinum catalyst in a thermoplastic silicone resin or an organic thermoplastic as described, for example, in EP-A-363 006. However, such techniques are relatively complicated.
A third possibility is to select, as catalyst, specific platinum complexes whose activity is such that the hydrosilylation reaction proceeds sufficiently rapidly at elevated temperature but at room temperature proceeds only to such a small degree that pot lives of a number of months are achieved. Addition-crosslinking compositions comprising such platinum complexes have been described, for example, in EP-A-583 159 and DE-A-36 35 236. Although the compositions described have significantly improved pot lives at sometimes sufficiently high crosslinking rates, there continues to be a need to improve the pot life and crosslinking rate of one-component, addition-crosslinking compositions by means of better platinum catalysts without having to accept the abovementioned disadvantages. This object is achieved by the present invention.